In a compact dis (will be referred to as “CD” hereunder) for example, one frame (588 channel bits) is composed of 32 symbols and each frame has stored therein a frame sync signal for detection of the head of the frame. In CD, data going to be recorded undergoes 8-14 (eight-to-fourteen) modulation (=EFM). In EFM, the maximum run length is 10. Therefore, the frame sync signal is formed from a pattern not included in EFM, more specifically, pattern of 11T and 11T' (the apostrophe (') indicates an inversion) and a pattern of 2T (100000000001000000000010). The sync signal is also used for a rotation servo control to drive CD at a CLV (constant linear velocity). That is, the patterns 11T and 11T' in the sync signal are longest ones in signals recorded in CD similarly to the maximum run length (that is, “11”) in EFM. On this account, the rotation servo controller detects the patterns, measures the length of them using a reference clock, adjusts the CD rotation velocity for the measured length to correspond to a reference length of time, and makes a PLL (phase-locked loop) control.
Some of the optical discs such as a CD, digital versatile disc (DVD) and the like have unique identification data such as a serial number recorded therein. The unique identification data is recorded in a predetermined area by a modulation method similar to that used for recording other data. The unique identification data is read from an optical disc being rotated at a CLV, for example, as in reading of the other data. In addition to the use for identification of an optical disc, the unique identification data can be used as play limitation data for allowing content data to be read only when it is desired to read the unique identification data, for example.
Note here that an optical disc such as content data recorded therein such as a CD, DVD or the like records digital data and thus permits to easily copy the data read from the optical disc to another recording medium without any deterioration of the data. Therefore, even in a system in which the unique identification data is used as such a play limitation data to allow or inhibit reading of content data, if the content data is copied as it is to a recordable medium, the unique identification data will also be copied like the other data. Namely, the unique identification data will also be recorded to an optical disc to which the content data is illegally copied, with a result that in the same system, a player can read the unique identification data from the optical disc to which the content data has illegally been copied and can play the optical disc to which the content data has illegally been copied. Thus, if the content data is illegally copied, it will hamper the possible interest of the copyright holder.
To avoid the above, some of the optical discs have provided in a part of the recording area thereof an identification data recording area for recording such unique identification data. The identification data recording area has formed therein pit patterns identical in interval of inversion to each other, and has unique identification data thermally recorded on the reflective layer to overlap the pit patterns. When reading the unique identification data from the identification data recording area, the rotation servo controller to control the rotation of the optical disc will take the pit patterns identical in inversion interval to each other as a frame sync signal and thus be disabled to rotate the optical disc at a normal velocity of rotation.